In the standardization of cellular communication technology of the fourth generation (4G) including LTE-A (Long Term Evolution-Advanced), MU-MIMO (Multiple User-Multiple Input Multiple Output) as further development of SU-MIMO (Single User-Multiple Input Multiple Output) is receiving attention. MU-MIMO is a technology enabling communication of a plurality of users in the same band simultaneously by spatial multiplexing and is expected to significantly improve communication throughput (see, for example, Non-Patent Literature 1 below).
SU-MIMO is used in LTE (Long Term Evolution), which is also called the cellular communication technology of the 3.9-th generation. In SU-MIMO, a base station selects a transmission weight to be used for pre-coding of down-link transmission in accordance with channels to individual base stations. However, it is difficult for the base station alone to know the channel to a mobile station and thus, feedback about the channel is given from the mobile station to the base station. One feedback method that can be used in LTE is the Implicit Feedback method. In the Implicit Feedback method, a predetermined number of transmission weights are prepared in advance. Then, an index of one transmission weight most suited to the channel estimated by the mobile station using a reference signal from the base station is fed back from the mobile station to the base station.